Fix NPE in removeGeofence

Check if geofence is null before reading getRadius()

Test: run cts-dev --module CtsLocationTestCases --test android.location.cts.LocationManagerTest
============================================
================= Results ==================
=============== Consumed Time ==============
    arm64-v8a CtsLocationTestCases: 1m 21s
Total aggregated tests run time: 1m 21s
============== TOP 1 Slow Modules ==============
    arm64-v8a CtsLocationTestCases: 0.29 tests/sec [24 tests / 81579 msec]
============== Modules Preparation Times ==============
    arm64-v8a CtsLocationTestCases => prep = 4604 ms || clean = 750 ms
Total preparation time: 4s  ||  Total tear down time: 750 ms
=======================================================
=============== Summary ===============
Total Run time: 1m 28s
1/1 modules completed
Total Tests       : 24
PASSED            : 24
FAILED            : 0
============== End of Results ==============
============================================


Bug: 134894083
Change-Id: I71f643688855394e14974170bd2380f66cd51308